"He had surgery this morning," Mourinho told a news conference. "I don't want to be very specific, I would prefer the doctor to give you some information officially.

"It's not the end of his season, not at all. It's a small intervention in his external meniscus and I want to believe that by the end of March he can be back.

"And if he can be back by the end of March, I hope we have a big April and a big May still to play in more than one competition."

The Belgian, who is out of contract at the end of the season, suffered the injury during the 2-0 defeat to Tottenham at Wembley on Wednesday night.

A day earlier, Mourinho had joked that Fellaini should "sign the contract and shut up" -- before he knew the extent of the injury -- instead of risking his future by running down his current deal.

Sources have told ESPN FC Fellaini turned down a contract offer from United in September. He has been able to negotiate a pre-contract agreement with clubs outside England since Jan. 1 amid interest from Turkey and Italy.

Mourinho has said repeatedly he would like him to stay but sources said the Belgian is looking for a longer deal than is currently on the table.

Rob is ESPN FC's Manchester United correspondent. Follow him on Twitter @RobDawsonESPN.